PROJECT SUMMARY/ABSTRACT: Pulmonary arterial hypertension (PAH) is characterized by excessive proliferation of apoptosis-resistant pulmonary artery endothelial cells (PAEC), smooth muscle cells (PASMC), and adventitial fibroblasts (PAAF) leading to progressive increases in pulmonary vascular resistance, and ultimately right heart failure and death. Recent studies suggest that pulmonary arterial (PA) stiffness is associated with increased mortality in patients with PAH; however, the mechanisms involved in the pathogenesis and progression of PA stiffening in PAH have yet to be fully elucidated. We have discovered that distal vascular matrix stiffening develops early in models of pulmonary hypertension (PH) and triggers a local mechanobiological feedback loop that amplifies vascular remodeling and accelerates disease progression. These findings suggest that PA stiffness is not merely a consequence of pathologic changes in the vessel wall, but can itself drive abnormal cellular behavior. We have identified YAP and TAZ as pivotal regulators of stiffness-dependent PASMC and PAEC mechanoactivation in PAH. Our preliminary findings suggest that mechanosignaling via YAP/TAZ drives vascular cell activation through suppression of cyclooxygenase (COX)-2-derived prostanoid production and bone morphogenetic protein (BMP) signaling. Silencing of YAP/TAZ in PASMC and PAEC abrogates stiffness- dependent increases in proliferation, matrix deposition, and traction force generation, and rescues suppression of prostanoid production and BMP signaling. Moreover, PASMC overexpressing a mutant TAZ that localizes constitutively to the nucleus have a dramatic reduction in COX-2-derived prostanoid production and BMP signaling leading to a hyperproliferative remodeling phenotype. We hypothesize that YAP/TAZ are activated by the mechanical environment to drive pro-remodeling cellular responses and promote matrix stiffening in an adverse feedback loop via suppression of prostanoid production and BMP signaling in PAH. In Aim 1, we will investigate the role that YAP/TAZ play in regulating COX-2-dependent prostaglandin production and vascular responses to matrix stiffening in PAH. We will determine the mechanisms of YAP/TAZ-dependent suppression of COX-2 and regulation of stiffness-dependent contractility and matrix synthesis in human PASMC, PAEC, and PAAF. In Aim 2, we will examine the mechanisms by which YAP/TAZ control TGF-! and BMP-dependent Smad signaling, suppress Id1 expression, and regulate cellular growth responses to BMP signaling in PAH. We will determine the impact of YAP/TAZ activity on proliferation, apoptosis resistance, and migration in PASMC, PAEC, and PAAF from PAH patients with and without BMPR2 mutations. In Aim 3, we will determine whether inactivation of YAP/TAZ arrests PA stiffening, attenuates vascular remodeling, and prevents right ventricular (RV) dysfunction in experimental PH. We will use murine models and pharmacologic approaches to modulate YAP/TAZ activity and assess the effects of YAP/TAZ inactivation on PA stiffening, hemodynamics, RV dysfunction, vascular remodeling, and regulation of the prostanoid and BMP pathways in models of PH.